MARYLAND GEOLOGICAL SUBVEY 33 



noted certain well-defined terraces which Avere distributed over the entire 

 region, and spoke of them as follows : 



" There is a practically continuous series of terraces and beach marks 

 along the fall line from the Eoanoke to the terminal moraine— a series 

 of shore lines as distinctive and unmistakable as those circumscribing the 

 valleys of the extinct lakes of the Great Basin, of India, of northern 

 Arabia, or of the partially ice-bound basins of Minnesota, Michigan, 

 Ohio, and New York, though they are generally more profoundly modified 

 by erosion and are frequently concealed by forests. These shore lines 

 embody an easily interpreted record of geologic vicissitude which coin- 

 cides in every detail with that of the Columbia deposits. They are 

 sometimes carved out of the sub-terrane, but are generally built of the 

 loam, sand, and gravel of which the Columbia formation consists and 

 are evidently coeval therewith. Now it is evident that these terraces are 



water fashioned; but they are not fluvial The forces concerned 



in the formation of the Middle Atlantic slope terraces acted horizontally 

 over great distances and with uniform energy for a considerable period, 

 filling depressions, softening contours, and obliterating relief, yet so 

 gently that essential homogeneity of deposit in the horizontal direction 

 and essential uniformity in surface prevails for miles. Only the un- 

 dulatory and horizontally acting force of waves appears competent to 

 produce so great expanses of uniform surface and constant structure as 

 are exhibited in this region." 1 



A summary of McGee's views regarding the various land movements as 

 expressed by the present state of the Lafayette and Columbia deposits 

 is as follows: 



Post-Chesapeake elevation and erosion. 



Lafayette (Pliocene ?) depression and deposition. 



Post-Lafayette elevation and erosion of at least 500 



feet; present topography defined. 



Early Columbia (Pleistocene) depression of about 200 feet and depo- 

 sition. 



Post-Early Columbia elevation and erosion. 



Later Columbia depression of about 100 feet and depo- 

 sition. 



x Amer. Jour. Sci., vol. xxxv, 1S8S, pp. 387-388. 

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